Common rail injection systems are already known. These are injection systems for internal combustion engines in which a high-pressure pump brings the fuel to a high pressure level. The pressurized fuel fills a pipeline system which is constantly pressurized during operation of the engine.
A common rail injection system of this kind is known from DE 10 2006 023 470 A1. The system described there has a high-pressure fuel pump for conveying fuel, a high-pressure fuel tank, connected to the high-pressure fuel pump, for storing fuel at injection pressure with respect to the environment of the common rail injection system, an injector, connected to the high-pressure fuel tank, for releasing the fuel into at least one combustion chamber, a return line for returning fuel from the injector to the high-pressure fuel pump at a return pressure with respect to the environment of the common rail injection system, and adjusting means for adjusting the return pressure. During operation the high-pressure fuel pump sucks in fuel from a fuel tank and compresses it to an injection pressure.
A further common rail injection system is known from DE 10 2006 026 928 A1. The system described there includes a fuel tank, a high-pressure fuel pump, a rail pipe, a pressure accumulator, an injector and a digital controller. A volume flow rate control valve, which is actuated by the digital controller via a volume flow rate control valve actuating line, is arranged in the feed line between the fuel tank and the high-pressure fuel pump. The high-pressure fuel pump comprises at least one compressor, for example a pump cylinder or plunger. During operation of the injection system it supplies a time-dependent injection pressure that is applied in the rail pipe to the injector.
In common rail injection systems the fuel is frequently supplied to the high-pressure fuel pump using a prefeed pump. This prefeed pump is usually arranged in the fuel tank and/or the supply pipe. It can be an electrically actuatable prefeed pump or a fixed delivery pump. By means of a prefeed pump of this kind the fuel is brought to a pressure value which is kept constant as a function of the volume of fuel removed by the high-pressure fuel pump using a pressure-limiting valve arranged between the fuel tank and the high-pressure fuel pump.
In practice prefeed pumps comprise manufacturing-related series variations. Furthermore, variations in on-board voltage supply occur in practice in the on-board supply system of the respective vehicle. The consequence of this is that different prefeed volumes are established in the case of electrically actuated prefeed pumps. This has the drawback that the fill level of the high-pressure fuel pump varies, and this can lead to problems in controlling the rail pressure. Furthermore, a volume of fuel that equalizes the range of variation must be stored in the case of electrically actuated fuel pumps, i.e. more fuel than is required has to be conveyed unnecessarily in order to avoid potential deficiencies in supply.
Increased control input is required in order to avoid different fill levels of the high-pressure fuel pump. Storing a volume of fuel that equalizes the range of variation is associated with the drawback of an increased power requirement and a possible over-dimensioning of the pump. Fuel consumption and the carbon dioxide emissions of the respective motor vehicle are also increased as a result.